1. Field of the Invention
The present invention is related to an optical amplifying apparatus for optically amplifying wavelength division multiplexed signal light produced by that a plurality of signal light having different wavelengths from each other is multiplexed in a wavelength division multiplexing manner, and also is related to an optical network apparatus with employment of this optical amplifying apparatus.
2. Description of the Related Art
The wavelength division multiplex transmission technique is such a technique required to increase a transmission capacity. The wavelength division multiplex transmission is suitable to increase the transmission capacity, but also to construct an optical network capable of switching route setting operations every wavelength channel. An optical network corresponds to such a network that while each of wavelengths is handled as a channel, an apparatus called as an optical ADM (optical add drop multiplexer) is employed, so that signal light is inserted into a transmission path every wavelength channel, or is branched from the transmission path.
However, in such an optical network, a wavelength channel is inserted, or branched, otherwise the number of optical transmitter module for transmitting a signal is changed in each of modes. As a result, a total channel number of signal light in the transmission path may be possibly changed. When the total channel number of the signal light transmitted in the transmission path, a level of channels which are continuously transmitted is varied. As a result, there are such problems that the transmission characteristic is deteriorated, or optical surge is produced.
The deterioration of the transmission characteristic caused by the change in the channel number becomes remarkable when a total number of wavelengths to be division-multiplexed, namely a total number of signal light is increased. In a prospective wavelength division multiplexed signal light transmission, when multichannel trend is progressed, the above-described problem will become more serious.
In this case, the generation process of the above problem in the optical network will be simply explained.
Generally speaking, as a basic control method for an optical amplifier, there are two control methods, namely a control method in which pumping light outputted from a pumping light source of an optical amplifier is controlled by way of the ALC (automatic level control) system so as to make the pumping light constant, and another control method in which pumping light outputted from a pumping light source is controlled by way of an AGC (automatic gain control) system so as to make a gain constant.
A first description is made of such a case that the gain of the optical amplifier is controlled by using the ALC system. When a total channel number of signal light to be transmitted is changed, the gain of the optical amplifier is controlled in such a manner that the output of this optical amplifier becomes constant by the ALC system. Otherwise, the gain of this optical amplifier is clamped to the maximum gain value. However, in any one of these cases, the output per 1 channel is changed.
Accordingly, in order to suppress such a change in the total channel number when the gain is controlled in the ALC control system, there is another control arrangement that the information about the total channel number is supplied as the monitor signal to the optical amplifier so as to control the gain. However, in this control system, since the gain is not controlled in synchronism with the change in total channel number, there is another problem that the level produced immediately after the total channel number is change cannot be compensated.
"Theory of Optical Amplifier Chains" Journal of Lightwave Technology, Vol. 16, No. 5, May 1998 (prior art publication 1) describes the transition response of the output from the optical amplifier in such a system that the optical amplifiers are relayed in the multi-stage manner and are controlled by way of only the ALC control system when the input levels of the optical amplifiers are changed. In this prior art publication 1, when the total channel number is changed in the transmission system that the optical amplifiers are relayed in the multi-stage manner, the transmission characteristic of the channel having the wavelength ".lambda.1" corresponding to the existing channel is considerably deteriorated.
Also, in such a case that a gain of an optical amplifier is controlled by employing the AGC control system, a level of a remaining channel is varied when a total channel number is changed. This channel level change is described in, for example, "Experimental and Theoretical Analysis of Relaxation-Oscillations and Spectral Hole Burning Effects in All-Optical Gain-Clamped EDFA's for WDM Networks", Journal of Lightwave Technology, Vol. 16, No. 4, April 1998 (prior art publication 2).
"Fast Link Control Protection For Surviving Channels in Multiwavelength Optical Networks" in Proc. 22nd European Conference Optical Communication, ECOC'96, Oslo, Norway, 1996, postdeadline paper ThC. 3.6 (prior art publication 3) discloses the following arrangement functioning as the technique capable of suppressing the level change in the existing channel when the total channel number is changed. That is, the output compensating channel having the different wavelength from that of the transmission signal channel is provided at the output of each node, so that the output from the node is kept constant.
However, the above-described prior art technique owns the below-mentioned problem. That is, first of all, in the transmission path of the optical network, in order to compensate for the level change in the existing channel when the total channel number is changed, if the adjusting signal light is provided with employment of such a new wavelength band, then there is a problem that the limited wavelength band of the optical amplifier is further limited.
As a second problem, in order to compensate for the level change in the existing channel in the transmission path, if the signal light having the different wavelength from that of the transmission channel is employed, it is required to employ an LD (laser diode) capable of stabilizing the levels of the respective channels.